IT'S likely to be a hazy weekend, as winds from the north-east blow smoke from hotspots in peninsular Malaysia.
Air quality measure the Pollutant Standards Index has been in the moderate range since Tuesday evening.
In an advisory issued on Friday, the National Environment Agency said: "We expect air quality levels in Singapore to fluctuate over the next 24 hours. 24-h PM2.5 concentrations may reach unhealthy levels during some parts of the day, although the situation is expected to gradually improve overnight.
"NEA will continue to monitor the situation and provide updates as necessary."
As a precaution, it advised people with chronic lung or heart conditions to avoid prolonged or strenuous outdoor physical exertion.
If the air quality worsens, the general population may need to reduce prolonged or strenuous outdoor physical exertion.
At 7pm on Friday, the 24-hour PSI remained moderate at 51 to 66 and the 24-hour PM2.5 concentration - which measures finer particles - was 36 to 55 micrograms per cubic metre at various places around the island.
The NEA blamed the haze on hotspots in southern Johor. There were 59 hotspots detected in peninsular Malaysia on Thursday and 35 in Sumatra.
Malaysia, too, has had poor air quality in the unhealthy to hazardous range. Environment and Water Resources Minister Vivian Balakrishnan said in a Facebook post on Friday: "We will continue to offer to assist the Malaysians as well."
Meanwhile, Friday afternoon saw very light rain at Jurong and Tuas, though this did not mean an end to the dry spell, the NEA said.
Warm and hazy conditions are expected on Saturday with brief and localised showers in the late afternoon over northern and western parts of Singapore.